Rapid consolidation of nanostuctured WC-FeAl3 by pulsed current activated heating and its mechanical properties

In the case of cemented WC, Ni or Co is added as a binder for the formation of composite structures. However, the high cost of Ni or Co, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC-FeAl₃ with a relative density of up to 97% was obtained within 2 min by PCAS under a pressure of 80 MPa. The average grain sizes of the sintered WC and WC-FeAl₃ were lower than 100 nm. The addition of FeAl₃ to WC enhanced the mechanical properties and increase of relative density.     

Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Relationship between hardness and fracture toughness in WC–FeAl composites fabricated by pulse current sintering technique

WC–FeAl composites having notable mechanical properties as hard metal materials are successfully fabricated by using pulse current sintering technique. The relationship between hardness H_V and fracture toughness K_IC of the composites is compared with those of WC–Co materials reported. The comparison suggests successful development of WC–FeAl composites with the characteristics almost equal to WC–Co materials currently used for the hard metal tools. Microstructure of the sintered WC–FeAl is uniform without any grain growth. In the K_IC–H_V plot, WC–FeAl composites locate at the same position as that of WC–Co materials. The total mechanical property of WC–FeAl is comparable to WC–Co. The developed WC–FeAl is a very promising candidate to replace WC–Co materials. Fully controlled microstructure is crucial.     

Effect of η-phase and FeAl composition on the mechanical properties of WC–FeAl composites

This study investigated the effects of η-phase generation and the binder phase composition of iron aluminide (FeAl) on the mechanical properties of tungsten carbide–iron aluminide (WC–FeAl) composites. The η-phase was generated in the composites when the WC–FeAl powder was highly oxidized before sintering in a vacuum. The presence of the η-phase resulted in the degradation of mechanical properties such as transverse rupture strength (TRS) and indentation fracture toughness (IFT). In addition, FeAl composition varied mainly as a result of α-Al₂O₃ formation. The iron-rich side of FeAl in binder phase improved the TRS and IFT. It was concluded that control of the η-phase generation and an appropriate FeAl composition are very important to maintain high TRS and IFT in the WC–FeAl system.     

Interface characteristics in cemented carbides with alternative binders

Cemented tungsten carbides with Co, Ni or Fe binders were studied by transmission electron microscopy. Particular attention was paid to phase and grain boundaries. A striking feature is the high frequency of coherent carbide/binder interfaces with Fe. The Fe rich binder adopts an epitaxy orientation relationship with prismatic facets of WC, with a parametric misfit of about 1.5%. A special orientation relationship with basal facets of WC grains is sometimes observed with Ni binder, as already noticed for Co. It is associated with a parametric misfit of about 15%. Binder segregation in WC grain boundaries was studied taking into account the effect of carbon content in the alloys. Whatever the binder, no influence of the carbon content could be pointed out. The analyses performed in random grain boundaries in WC-Co alloys agree with the literature value of 0.5 monolayer of segregated Co while slightly larger values are obtained for Ni and Fe binder. ∑ = 2 special grain boundaries were studied in WC-Co and WC-Ni alloys and no segregation was detected. The higher grain boundary segregation as well as the occurrence of coherent interfaces should influence the mechanical properties of WC-Fe alloys.     

Highly dense,inexpensive composites via melt infiltration of Ni into WC/Fe preforms

Traditionally, WC-based composites use Co as the metal binder phase to consolidate using liquid-phase sintering with a small percentage of Co, but a potentially lower cost binder phase can be made with a different approach when using large amount of metal binder phase. FeNi as a metal binder material is much cheaper than Co. WC can be liquid-phase sintered and melt infiltrated with FeNi, but by making FeNi in situ, costs lower even further. Composites of WC-(Fe-Ni) were made by first pressing a mixture of WC and Fe powders and subsequently melt infiltrating Ni in an amount corresponding to less volume than the porosity of the preform to ensure high WC content. The research objective was to make highly dense composites via melt infiltration with a low-cost metal binder phase in situ. This method has the potential to make fully dense composites, rather than hardmetals, with suitable properties at lower costs. The density and hardness are 97.4%TD and 6.72 GPa, respectively.     

Fabrication of cemented tungsten carbide components by micro-powder injection moulding

Micro-powder injection moulding (micro-PIM) is an advanced net-shaping process for the fabrication of metal and ceramic complex micro-components. Cemented tungsten carbide (WC–Co) hardmetal is known for its high hardness and wear resistance in various applications. Micro-PIM is a new alternative manufacturing technique for hardmetal micro-parts. In this work, the fabrication of WC–Co components via a micro-PIM process was studied. A fine WC–10Co–0.8VC (wt.%) powder was mixed with a binder system consisting of paraffin wax, low density polyethylene and stearic acid. A micro-component was injected at low pressure using a semi-automatic injection moulding machine. The injection temperature was determined from the rheological investigation of the feedstock. The binder extraction was carried out in solvent and thermal debinding methods under an argon atmosphere. Thermal gravimetric analysis was used to confirm the removal of the soluble binder from the green part. The sintering process has been performed within a temperature range of 1330–1450 °C under vacuum. After sintering, a density of 94.5% theoretical density was obtained, which is a reasonable value. The micro-components showed length shrinkage between 16 and 22% and good surface quality and hardness values when compared with conventional powder metallurgy. This research shows that micro-PIM is able to produce small WC–Co components with properties comparable to conventional powder metallurgy.     

Ni3Al添加量对WC-Co硬质合金中WC晶粒形状的影响

采用粉末冶金制备技术,以粗WC粉末、Co粉和WC＋Ni3Al预合金粉末为原料制备出WC-40vol％（Co—Ni,Al）硬质合金。利用扫描电镜和透射电镜研究了不同NbAl含量对WC-40vol％（Co—Ni3Al）硬质合金中WC晶粒形状的影响规律。结果表明：W在Co粘结相中的固溶度接近25．4wt％,而W在Ni,Al粘结相中的固溶度接近9．5wt％,随着NbAl含量的增加,粘结相对W的固溶度减小,合金中的WC晶粒圆钝和细小;WC晶粒表面上出现明显的台阶。相应的,延长烧结时间,WC—Co—Ni3Al硬质合金具有与WC—Co硬质合金相同的WC生长行为,WC-40vol％（Co—Ni3Al）硬质合金中的WC晶粒表面上的台阶处出现明显的刻面。     

WC—8（Fe／Ni／Co）R硬质合金的研究

用Ｆｅ、Ｎｉ部分取代ＷＣ－８Ｃｏ合金（牌号ＹＧ８）中的粘结剂Ｃｏ,再添加微量稀土Ｒ制得ＷＣ－８（Ｆｅ／Ｎｉ／Ｃｏ）Ｒ硬质合金,测试了其物理机械性能,研究了粘结剂各种成份比例及烧结温度对硬质合金的影响。结果表明：ＷＣ－８（Ｆｅ／Ｎｉ／Ｃｏ）Ｒ硬质合金的性能可以达到ＹＧ８的性能标准。利用扫描电镜（ＳＥＭ）及Ｘ射线衍射（ＸＲＤ）对其作了显微结构分析,同时探讨了影响ＷＣ－８（Ｆｅ／Ｎｉ／Ｃｏ）Ｒ合金性能的     

碳化钨/金属间化合物硬质合金的研究进展

对目前国内外以金属间化合物Ni3Al、FeAl、Fe3Al为粘结相的碳化钨基硬质合金的制备方法和性能特点进行了综合评述,重点介绍了硬质合金国家重点实验室在以Ni3Al为粘结相碳化钨基硬质合金方面的最新研究成果及应用情况。结果表明:均匀细小的金属间化合物预合金粉末的制备以及适当控制界面反应是粉末冶金法制备碳化钨/金属间化合物硬质合金的有效方法。碳化钨/金属间化合物硬质合金的研究方向应集中在界面问题、金属间化合物粘结相的韧化、制备工艺和综合性能评价体系的建立等几个方面。     

Carbide grain growth in cemented carbides sintered with alternative binders

Cemented carbides are composites made of a hard refractory ceramic phase and a ductile binder, most commonly WC and Co, respectively. Since the use of cobalt in the hard metal industry is questioned by the new European regulation on chemicals, extensive research has been done to develop new grades based on a Co-Ni-Fe binder. With similar mechanical, physical properties and affinity to C and W, nickel and iron are the best candidates for an efficient binder in cemented carbides. As mechanical properties are strongly dependent on the materials microstructure, and especially on the WC grain size, understanding the effect of the binder on the final microstructure is crucial.In this work, the carbide grain growth behaviour of WC-M alloys (M = Co, Ni, Fe) with different carbon contents is discussed from qualitative and quantitative microstructural analyses. Whereas grain growth is more or less inhibited in WC-Fe alloys, increasing carbon content promotes grain growth in WC-Co and WC-Ni alloys, with a slight abnormal grain growth in case of Ni binder. Different mechanisms for grain growth are discussed, in relation with the observed morphology of WC grains after sintering.     

Investigation of the corrosion behavior of WC–FeAl–B composites in aqueous media

The aim of this work is to investigate the corrosion mechanisms of WC–FeAl–B composites, with different amounts of boron (0, 250, 500, 750 and 1000 wppm) in acidic, neutral and alkaline solutions using electrochemical methods and solution analysis. Results obtained showed that the pH of the solution significantly affects the corrosion susceptibility. In acidic solution, the corrosion process of WC–FeAl–B consists mainly of Fe dissolution, and the addition of boron of more than 500 ppm increases the corrosion rate. Synergistic effects due to galvanic coupling between the FeAl–B binder and WC accelerated the dissolution of binder.     

Characterisation of wear properties of ultrafine-grained hardmetals using a special abrasive wheel test

Three-body abrasive wear tests were carried out on super ultrafine-grained hardmetals (WC intercept length about 0.2 μm) with different Fe, Ni, Co binder systems, on hardmetals with Co binder and on standard materials with coarser WC grains. Apart from the standard materials, the hardmetals were all produced in laboratory scale from commercial nanocrystalline WC powders from different companies. The practical application of these hardmetals is in the field of cutting dry wood. The main purposes of this work were to characterise the wear properties of the produced grades and to derive predictions from laboratory wear tests regarding practical applications. Therefore a new wear test apparatus was built. The wear results showed a logarithmic correlation with hardness and a threshold to the low wear region, which was exceeded by some grades, at binder mean free paths of less than 40 nm. Moreover the laboratory wear results correlate well with results from field testing. A tribofilm was found on the worn surface and characterised by scanning electron microscope (SEM), AFM and TEM. In an analysis of the wear mechanisms the role of this tribofilm must be considered.     

Microstructure and composition of the grain/binder interface in WC–Ni3Al composites

The microstructure and composition of WC/Ni₃Al interface were studied. An orientation relationship of [100]_WC//[110]_Ni3Al and (001)_WC//(001)_Ni3Al with a good coherence, besides many random orientation relationships between WC and Ni₃Al, has been repetitively found by selected area electron diffraction and high resolution TEM observations. The XRD pattern of WC–Ni₃Al composites indicated that the major binder phase was Ni₃Al and showed possibility of coherence between WC and Ni₃Al as common interplanar spacings existed. Electron probe microanalysis results revealed that the atomic ratio of Al:Ni is close to 1:3 in binder phase and WC/Ni₃Al interface in the WC–Ni₃Al composites has a sharper compositional gradient and a smaller width of transition region than the WC/Co interface in WC–Co composites.     

用超合金作粘结相的硬质合金的TEM研究

用透射电子显微镜研究了由一种Ni基超合金作粘结相的硬质合金的组织结构。结果表明，烧结后合金的粘结相γ中分布着高度弥散的γ＇相。γ＇相质点在位错网络的结点上择优形核，形貌有粒状和片状两种，大小为10um数量级。观察表明，粘结相γ与WC的湿润性良好，因此用Ni基超合金作硬质合金的粘结相，不仅可节约战略物质Co，而且有可能使硬质合金的性能提高，使用范围扩大。     

超音速火焰喷涂WC-10Co4Cr涂层的耐滑动磨损行为

采用超音速火焰喷涂(HVOF)工艺制备微米结构WC-10Co4Cr涂层,分别采用金相显微镜、扫描电镜(SEM)、X射线衍射(XRD)和滑动磨损设备分析涂层的微观结构和滑动磨损行为。结果表明:采用液体煤油燃料HVOF喷涂的微米结构WC-10Co4Cr涂层的脱碳程度较低,涂层中仅出现WC和W2C相,而无η相(Co3W3C、Co6W6C)以及软相W。涂层微观结构致密,孔隙率约为1%,平均显微硬度为1 322HV0.3;在相同试验条件下,WC-10Co4Cr涂层的摩擦因数(约0.8)高于不锈钢(1Cr18Ni9Ti)的摩擦因数(约0.5),其滑动体积损失量仅为不锈钢涂层的1/146,具有优异的抗滑动磨损性能。涂层在滑动磨损过程中首先是粘结相的脱落,然后是WC颗粒的磨损。     

Fabrication and mechanical properties of FeAl/TiC composites

1　INTRODUCTIONOver the past few decades, considerable inves tigations have been carried out to identify alterna tive binders for cermets in order to improve theirmechanical properties and also to overcome certainshortcomings, such as high cost and density, lowoxidation and corrosion resistance, and environ mental toxicity[1, 2]. Iron aluminides are of particu lar interest due to their low cost and density, highspecific strength, environmental friendliness andexcellent oxidati…     

Tungsten carbide coatings with different binders prepared by low power plasma spray system

Thermal spraying of cermet coatings is widely used for protection of machining parts against wear and corrosion. These coatings consist of WC particles in metal binders such as Co, Cr and Ni. Three kinds of WC powders with different metal binders (Co, NiCr and CoCr) were sprayed by low power plasma spray system on Al-Si-Cu alloy substrate. Fundamental aspects of sprayed cermet coatings, including (i) the effects of binder type on the coating structure, (ii) the hardness and (iii) the microstructure, were investigated. All cermet coatings have the same phase structure such as WC and W2 C. However, the intensities of these phases are different in each coating, mainly due to the difference in solidification rate in each case. Moreover, the hardness measurements are found to be different in each coating. The results show that, binder type has a significant effect on the physical and mechanical properties of the sprayed coatings.     

无压渗透制备金属间化合物陶瓷基复合材料的研究进展

介绍了目前国内外使用无压渗透制备金属间化合物陶瓷基复合材料几个系列：FeAl／TiC、NiAl、Ni3AL／TiC、FeAl/WC和Ni3Al/WC。     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.